The field of the invention is input circuits to digital control systems such as programmable controllers, numerical control systems, and process controls, and particularly, to input circuits which convert the signals from various types of sensing devices into digital logic signals.
Digital control systems are commonly connected to sensing devices on industrial machines such as limit switches, pushbutton and selector switches, pressure switches and photoelectric switches. In a programmable controller system, for example, hundreds or even thousands of such sensing devices may connect to an I/O interface system which provides a separate input circuit for each device.
Each input circuit performs three primary functions. First, it converts the signal received from the sensing device to a logic level signal which is compatible with the digital control system. Second, it provides electrical isolation of the control system electronics from the surrounding environment, and third, it provides filtering which immunizes the control system from industrial noise and contact "bounce."
Because there are numerous types of sensing devices employed on industrial machines, a variety of input circuits must be offered by the controls manufacturer to interface his digital control system with the user's machine. For example, separate input circuits must be provided to interface with 220 volt a.c. signals, 120 volt a.c. signals, 42 to 53 volt d.c. signals or 10 to 26 volt d.c. signals. As a result, there is no known standard input circuit and it is common practice in the art to provide a set of input circuits built from discrete components for each particular digital control system offered by the manufacturer.